U.S. presidential candidate and former Secretary of State Hillary Clinton participates in a discussion in a classroom at New Hampshire Technical Institute while campaigning for the 2016 Democratic presidential nomination in Concord, New Hampshire, April 2

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The Clinton Foundation's acting chief executive admitted on Sunday that the charity had made mistakes on how it listed government donors on its tax returns and said it was working to make sure it does not happen in the future.

The non-profit foundation and its list of donors have been under intense scrutiny in recent weeks. Republican critics say the foundation makes Hillary Clinton, who is seeking the Democratic presidential nomination in 2016, vulnerable to undue influence.

After a Reuters review found errors in how the foundation reported government donors on its taxes, the charity said last week it would refile at least five annual tax returns.

"So yes, we made mistakes, as many organizations of our size do, but we are acting quickly to remedy them, and have taken steps to ensure they don't happen in the future," Clinton Foundation acting Chief Executive Officer Maura Pally said in a statement.

The errors appeared on the tax forms 990 that all non-profit organizations must file annually with the U.S. Internal Revenue Service to maintain their tax-exempt status.

Pally said the foundation did accurately report its total revenue but government grants were mistakenly combined with other donations.